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The large magma reservoirs underlying world-class porphyry deposits are one of the key factors in their formation, which thus led to the present study focusing on the unmineralized intrusive rocks underlying the porphyry molybdenum-copper orebody of the post-collisional Jiama porphyry-skarn copper-polymetallic deposit in southern Tibet. The Jiama porphyry intrusion comprises intermediate-silica quartz diorite, monzogranite, and granodiorite porphyries, as well a high-silica granite porphyry. The intrusive rocks suite exhibits similar whole-rock Sr-Nd isotopic compositions [εNd(t) = -3.9 to -0.8], suggesting a common, non-radiogenic magma source. Yet, these rocks have distinct geochemical characteristics. The intermediate-silica rocks are relatively enriched in Ba and Sr with minor Eu anomalies, and show adakite-like geochemical characteristics. In contrast, the high-silica granite porphyry is strongly depleted in Ba, Sr, and Eu, and lacks adakite-like geochemical characteristics. We propose that the high-silica granite porphyry represents highly fractionated melt extracted from a silicic mush reservoir (crystallinity of ∼40%-65%), and that the monzogranite and granodiorite porphyries constitute the complementary residual silicic cumulates. High crystallinity facilitates the formation of connected fluid pathways, allowing the efficient removal of volatiles from the remaining silicic melt and a rapid flux of the ore-forming fluids toward the apical parts of the large magma reservoir. Less-evolved mafic melt is believed to have repeatedly intruded the base of the magma reservoir, thereby releasing volatiles and water into the silicic mush systems. The volatiles migrated upward through the fluid channels and accumulated in the apical part of the magma reservoir, subsequently, as a result of the overpressure in the roof zone, ore-forming fluids and successive batches of magma were expelled together, thereby forming the Jiama porphyry-skarn deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • Three types of magnetite were identified in the Mamupu deposit based on the detailed textural and mineral assemblage. • The study on texture and composition of magnetite restricts the evolution process of breccia ore body. • The geochemical characteristics of magnetite reveal the coupling relationship between Cu-Au and W-Mo in the Mamupu. The Mamupu copper polymetallic deposit is characterized by multistage magnetite, but the genesis and fluid evolution process of this deposit are still obscure. Three types of magnetite were identified in Mamupu based on comprehensive textural observations and paragenesis. Mag1 occurs as subhedral to euhedral crystals, and smooth Mag1a and porous Mag1b can be distinguished under Backscattered Electron (BSE). Mag2 is characterized by 120° triple junction textures and has been replaced by hematite. Mag3 is commonly replaced by hydrothermal fluids and exhibits well-developed oscillatory zoning and core (Mag3a)-rim (Mag3b) textures. The geochemistry of Mamupu magnetite suggests that they are of hydrothermal origin. Mag1 has undergone extensive fluid-rock interactions, whereas the sharp contacts with Mag1a and Mag1b and the microporosity of Mag1b imply that the Mag1b was generated by the coupled dissolution reprecipitation of the Mag1a grains. This process may be triggered by low-temperature and high-salinity fluid produced by intensive water-rock interaction between ore-bearing magmatic-hydrothermal fluid and carbonate surrounding rock of the Jiaoga Formation. The 120° triple junction textures of Mag2 indicate that it was formed during a fluid-assisted recrystallization process, and Mag2 replaced by hematite represents the increase in oxygen fugacity. From Mag3a to Mag3b, the contents of V and Mn increased while the contents of chalcophile elements decreased, suggesting a decrease of oxygen fugacity and the precipitation of sulfide, which represents the beginning of the sulfide stage. Moreover, petrography and geochemistry data of magnetite represented that the breccia ore of Mamupu belongs to skarn mineralization. The geochemical characteristics of magnetite show that Cu-Au and W-Mo in Mamupu precipitated in different stages, representing the transformation from a relatively oxidized environment in the early stage to a relatively reduced environment in the late stage. These results reveal that trace elements in magnetite can be utilized to define the genesis and evolution of the skarn deposit. In addition, it can also restrict the precipitation mechanism of ore-forming elements in the deposits. However, detailed textural observations should be carried out before experimental research. [ABSTRACT FROM AUTHOR]